Geometry Constructions

A series of free, online High School Geometry Video Lessons.
Videos, worksheets, and activities to help Geometry students.

In these lessons, we will learn

how to construct the circumcenter of a triangle

how to construct the orthocenter of an acute triangle and an obtuse triangle

how to construct the centroid or center of gravity of a triangle

Constructing the Circumcenter

The point of concurrency of the three perpendicular bisectors of a triangle is the circumcenter. It is the center of the circle circumscribed about the triangle, making the circumcenter equidistant from the three vertices of the triangle. The circumcenter is not always within the triangle. In a coordinate plane, to find the circumcenter we first find the equation of two perpendicular bisectors of the sides and solve the system of equations.
How to construct the circumcenter using a compass and straightedge.

Using compass and straigtedge to construct the circumcenter of a triangle.

Constructing the Orthocenter

The orthocenter is the point of concurrency of the altitudes in a triangle. A point of concurrency is the intersection of 3 or more lines, rays, segments or planes. Theorthocenter is just one point of concurrency in a triangle. The others are the incenter, the circumcenter and the centroid.
How to construct the orthocenter of a triangle.

Using a straight edge and compass to create the orthocenter of an acute triangle

Using a straight edge and compass to create the external orthocenter of an obtuse triangle

Constructing the Centroid

The centroid is the point of concurrency of the three medians in a triangle. It is the center of mass (center of gravity) and therefore is always located within the triangle. The centroid divides each median into a piece one-third the length of the median and two-thirds the length. To find the centroid, we find the midpoint of two sides in the coordinate plane and use the corresponding vertices to get equations.
How to identify the centroid and the way it divides each of the medians.

Using a straight edge and a compass to create the centroid or center of gravity of a triangle

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